Both actresses bared all on the big screen at an age when most women are doing their best to cover up their bodies.

Bullock had tongues wagging over her first nude scene in the new film "The Proposal," which debuted July 10 at the top of the box office.

"Box office like this, I'm going to be naked in every film," she joked on a British morning show.

Known for playing prim characters in films such as "Miss Congeniality" in 2000, Bullock is the latest actress older than 40 to jump on the nude bandwagon. Mary-Louise Parker, Marisa Tomei and Kathy Bates all waited until after 40 to shed their clothes on screen.

The reasons vary.

"They may need the money or need the work," Jeanine Basinger, the film studies chairwoman at Wesleyan University in Middletown, Conn., told ABCNews.com. "If you're a working actress of a certain age, you have to make a living.

"A lot of them say it's the part," she added. "In Bullock's case, that scene is a major comedy moment."

UCLA film professor Vivian Sobchack said, "It's also a way of reinventing oneself and reviving a career. It's an assertion of I'm still viable, I'm still here damn it, I'm still erotic and I still have sex appeal."

Whatever the case, the way today's actresses older than 40 look on screen and the way audiences look at them are a lot different from decades past.

"There is a larger phenomenon, in which 50 is the new 30," Sobchack said. "Within the context of our make-yourself-over culture, there is a very different attitude toward the notion of older women, particularly. No longer are they merely disgusting, or your mother.

"It used to be women at a certain point, over a certain age, were seen primarily as mothers or grandmothers -- certainly benign or bitchy, but really not sexual," she added. "That's why Mrs. Robinson was considered so sexual way back."

When actress Anne Bancroft, at 36, played the older Mrs. Robinson who seduces a younger man, played by Dustin Hoffman, in 1967's "The Graduate," she was seen as almost too sexy. She didn't work again for the next five years.

Tight Bodies Help With Confidence

Flash forward 40 years, to the era of Demi Moore and Ashton Kutcher, and where the term cougar -- the older woman in a relationship with a younger man -- has become hip.

"Cougar has a certain negative but exciting quality," Sobchack said. "It's a way to sort of acknowledge not only an older women's sexuality but their erotic power."

Think actress Kim Cattrall, a cougar in real life as well as on screen. At 52, she turned her body into a sushi platter for her younger lover in the "Sex and the City" movie.

"She's a great role model for being comfortable with yourself," blogger Melissa Silverstein, who runs the Web site Women and Hollywood, told ABCNews.com.

Of course, it helps that Cattrall, Bullock, Parker and Tomei are showing off toned and taut bodies.

"One might ask how many saggy over-40 bodies do you see on the screen?" Sobchak said. "It's not some form of liberation. It's a form of triumph over maintaining militant control of your body, having tummy tucks and everything else.

"Yes, we're seeing older women on the screen as erotic full-fledged women and not only that, they are looking good," Sobchak added. "The bad part is how many of us don't look that good. And, again, like most Hollywood images, we're not talking about realism here."

There are notable exceptions. In 2003's "Something's Gotta Give," Diane Keaton said she did nothing special to prepare for her first nude scene at 57. "I thought, 'It's going to be a woman's body and that's the way it is,'" she told About.com.

Keaton had also grown more comfortable with her body and no longer had the same hang-ups about showing it onscreen.

"Your idea about your body changes completely as you get older," she told Entertainment Weekly at the time the movie came out. "Now, I just see it as a body. It's not like this precious commodity that I have to hide because I'm, like, Omigod, I don't want anyone ever to see me ever-ever, which I felt for about a bazillion years."

Many audiences commented on how great Keaton's figure looked. Not so with Bates. When the actress, then in her mid-50s, flashed Jack Nicholson in "About Schmidt" in 2002, "It was a shock," Basinger said. "Bates was fabulous in this movie, but seeing her naked is a shock, because we are never confronted with the site of a real 40-something woman carrying some weight, unless we are looking in the mirror. Even Jack Nicholson's character was repelled by her."

Going Nude to Play the Role

Bates took her clothes off because the part called for it. Tomei said she did the same thing for "Before the Devil Knows You're Dead" in 2007, a year before she played a stripper in "The Wrestler," because she wanted to work with director Sidney Lumet, who insisted on explicit nudity for her character.

"I trusted Sidney," Tomei told the Los Angeles Times. "I trusted his taste, and that was a requirement of the role, and I wanted to work with him enough that I was willing to take that leap of faith."

Sobchak said Tomei's decision to strip in "The Wrestler" was a good career move because it got her noticed again. Blogger Silverstein, on the other hand, said the film overdid the nudity.

In the case of "The Proposal" and "Something's Gotta Give," Silverstein said the nudity served as humor in the film.

"It was something you could see happening," she said. "A lot of times where younger women were shown naked or semi-nude, it's gratuitous. When you get into a conversation with older women, it's not about pleasing the male eye or male desire, it's about the script."

While younger actresses, like Jessica Alba, Natalie Portman and Reese Witherspoon, have made it a point to swear off nude scenes, in part, because they worry about the images being widely circulated on the Internet, older actresses appear more at ease with the idea of showing skin.

But Parker, 44, publicly regretted going nude on screen for her Showtime drama series "Weeds."

"I didn't think I needed to be naked, and I fought with the director about it, and now I'm bitter," she told More magazine in June. "I knew it was going to be on the Internet: 'Mary-Louise shows off her big nipples.' I wish I hadn't done that. I was goaded into it."

The bottom line is the decision to undress rests with the actress. "I don't think you have to take your clothes off to be a star that shines for a very, very long time," Sobchack said. "That's not the criteria one way or the other."